I'm not a car audio expert but...

It's my understanding that a power lead would be to raise/lower a
"power antenna" and/or to supply power to an "amplified antenna." My
guess is that your stock-type (?) antenna is neither of these. So it
wouldn't seem to me that connecting it to power would change
reception. But, your "radio dude" could/should know something I don't.

Was the reception good before, and then suddenly got worse? Often
those adapters shake loose after time and just need reconnecting.

James V
86 Syncro Westy, etc.